This invention relates to a switched-mode power supply circuit comprising a series arrangement of an inductive element and a controllable switch coupled between the terminals of a DC input voltage. A rectifier is coupled to the inductive element for making a DC output voltage available at a load connected thereto and a control circuit is coupled to the switch for rendering the switch alternately conducting and non-conducting. A pulse duration modulator is coupled to the control circuit for determining the duration of the conductivity time of the switch. A function generator is provided for applying a signal, which is a given function of the output voltage, to a control input of the modulator for maintaining the output voltage at a substantially constant, first value which is independent of the values of the input voltage and of the load, and means are also provided for bringing the output voltage to a second value which is considerably lower than the first value by keeping the conductivity time of the switch shorter.
A power supply circuit of this type is known from an article in the German magazine "Funktechnik" 37 (1982), no. 1, pp. 21-25. It is apparent from this publication that the output characteristic of the known circuit, i.e. the diagram of the variation of the output voltage as a function of the output current flowing through the load, has a shape which is folded back (current fold-back characteristic). If the output current has a value between zero and a given maximum value, the output voltage has the first value so that the characteristic is a substantially horizontal straight line. This is ensured by the above-mentioned control of the duration of the conductivity time of the controllable switch. If the output current exceeds the maximum value, the conductivity time will be quickly reduced so that the output voltage will be considerably lower than aforesaid the first value (which holds for the nominal case) and by which the peak power is reduced to a value which is within safe limits for the load and for the supply. If the output voltage is zero, i.e. if the load is a short circuit, the output current is limited to a given, small value. The characteristic is thus folded back between the point in the diagram representing the maximum value of the current and the point corresponding to the short circuit. The supply circuit remains operative under all circumstances.
During the transition to the normal operating condition after switching on the power supply circuit, the output voltage increases to the nominal value because the successive conductivity periods of the switch become longer, the working point in the diagram being displaced from the origin of the system of coordinates to the point of the characteristic corresponding to the nominal value. If the output voltage has assumed a low value due to an overload, the working point is displaced towards the nominal position similarly to the case after switching on, unless the interference is still present, in which case the output voltage remains at the low value. The increase of the output voltage is slower than its decrease, thus ensuring that each time no large peak currents flow which would be a heavy burden for the components of the circuit. This enhances its reliability. During such a soft start both the output voltage and the output current increase simultaneously until the working point has reached the position on the characteristic curve which corresponds to the maximum value of the current, while the voltage substantially has the nominal value. Subsequently, the control ensures that the nominal condition is reached, i.e. the working point moves along the horizontal portion of the characteristic curve, while the current becomes smaller. It is apparent therefrom that during the start an unnecessarily large amount of energy is dissipated.